The goal of this research project is to develop small, inert gadolinium complexes which can be utilized as proton relaxation enhancers in Nmr imaging studies. Gadolinium complexes with acyclic planar tetradentate, pentadentate, and hexadentate ligands will be prepared. The synthesis of several different Gd macrocycles with 18-to-21 membered planar pentadentate, hexadentate, and heptadentate ligands will also be investigated. These nitrogen-containing macrocycles will be prepare by employing Schiff base condensation reactions with Gd(III) as the template. All complexes will be characterized in the solid state by infrared spectroscopy and elemental analysis and in solution by ultraviolet-visible spectroscopy and high performance liquid chromatography. The kinetic lability of the complexes in aqueous solution will be determined, and the time frame of metal-exchange and ligand-exchange will be assessed. The complexes will be prepared in low specific activity with 153Gd, the [153 Gd]-Gd complexes will be incubated in distilled water, in aqueous solutions containing metal ions, and in human serum. At selected time points, aliquots from the different solutions will be analyzed by HPLC with a radiometric detector, and the various 153Gd species present in solution will be identified. The in vitro and in vivo relaxation behavior of those Gd complexes which remain inert in solution will also be studied The T1 and T2 effects of the complexes at 10 MHz in water, 5% HSA, human serum, and whole blood will be determined. Normal rats and rabbits will be imaged at 80 MHz before and after the injection of the Gd complexes, and changes in in vivo contrast will be followed. Additional animal studies will also be conducted to better understand and quantitate the complexes' in vivo action and relaxation behavior. The [153 Gd]-Gd complexes will be injected at dose necessary for in vivo contrast, and at selected time points the animals will be sacrificed. Preliminary acute toxicity studies will be conducted, and veterinarian will perform gross and histopathologic examinations of the blood and organs. Blood and urine samples will be analyzed by HPLC to characterize the in vivo chemical form of the 153Gd, the activity in the organs will be counted to determine the approximate Gd concentration in targeted organs, T1 and T2 of excised tissue and blood samples will be measured at 10 MHz, and the relaxivity ratios and enhancement factors for the agents in targeted tissues will be calculated.